Tubular myelin is a form of pulmonary surfactant which has a unique lamellar structure, possibly resulting from a fusion of phospholipid bilayers at fairly regular, long-range intervals. This structure markedly affects the surface properties of surfactant, and materials of similar composition, but different morphology, function less satisfactorily as alveolar surfactants. Previous work has shown that an apolipoprotein specifically found in pulmonary surfactant, and Ca2+ ions, are required for this structure. The work in this proposal will investigate the physical properties of recombinant complexes formed between certain phospholipids and this apolipoprotein, with the overall goal of using this relatively simple system to obtain information that can be applied to the analysis of the more complicated natural surfactant. The experiments are developed around 4 broad objectives: (1) to determine if the apolipoprotein, upon binding lamellar lipids, will induce a lateral phase separation; (2) to characterize the physical state of the phospholipid in the immediate environment of the bound apolipoprotein; (3) to describe the secondary structure of the apolipoprotein in solutions, and to determine if it changes upon interaction with the phospholipid; and (4) to determine if the surface properties of the recombinants can be correlated with the observed differences in lamellar organization. These physical properties will be analyzed by the measurement of surface tension, steady-state fluorescence anisotropy, differential scanning calorimetry, and circular dichroism.